1. Field of the Invention
The present invention is directed to a rechargeable battery pack and charger unit combination.
2. Background Art
Such combination of a rechargeable battery pack and a charger unit has been widely utilized for power driven devices such as screwdrivers or the like portable tools. The charger unit includes a charging circuit which is controlled to supply a charge current to the rechargeable battery in the battery pack until the battery is fully charged. For a certain type of the rechargeable battery, it has been a practice to monitor the temperature of the battery being charged as an effective parameter indicating the terminal voltage of the battery in order to control the charging circuit. To this end, a temperature sensor is included in the battery pack for transmitting a sensor output indicative of the battery temperature, or the charge condition of the battery to the charging circuit for control thereof. As disclosed in the Japanese Utility Model Publication (KOKAI) No. 56-48241 published on Apr. 28, 1981, a prior charge control scheme based upon the battery temperature requires in the charger circuit a judge-and-hold circuit which judges from the output of the temperature sensor that the battery temperature or the terminal voltage exceeds a predetermined critical level to be indicative of the battery being fully charged, and which holds the charger circuit in an OFF condition of supplying no charging current to the battery once the fully charged condition is judged for prevention of the overcharging which would otherwise result as the battery temperature falls below the critical reference level.
In the meanwhile, there have been proposed in the art a wide variety of battery packs with rechargeable batteries of different voltage-temperature characteristics. In this connection, it is required to use different temperature sensors particularly compatible with the kinds of the rechargeable batteries selected for providing a reliable sensor output well indicative of the fully charged condition of the rechargeable battery. This means that the temperature sensors with the outputs of differing levels are used within the battery packs of different types. If this is the case, the charger unit is also required to incorporate the judge-and-hold circuit which is particularly designed in exact correspondence with the output level of the sensor selected in the particular battery pack. With this result, different types of the charger units may be required for different types of the battery packs. This is not economical in view of that different types of battery packs may be required for different power operated devices but a single type of the charger unit is enough for charging. Therefore, it is highly desired to utilize the battery charger common to the different types of the battery packs utilizing different temperature sensors.